Method of copper coating steel



United States Patent 3,311,493 METHOD OF COPPER COATING STEEL Calvin A. Schunemann, Gates Mills, Ohio, assignor to Horizons Incorporated, a corporation of New Jersey No Drawing. Filed Oct. 12, 1964, Ser. No. 403,327 Claims. (Cl. 11771) This application is a continuation-in-part of my copending application Ser. No. 296,385, filed July 19, 1963.

This invention relates to a novel composition of matter involving copper coated steel and to a process for producing corrosion resistant articles of such materials.

Ordinary steel, such as the grades normally produced in the basic open hearth process (e.g. SAE 1020), is one of the most versatile and economical of structural materials possessing high strength, hot and cold working properties, and many other properties which make it suitable for forming into a diversity of useful shapes including structural beams, angles, sheet, tubs and tubes. Unfortunately, the corrosion resistance of these articles is mediocre, if not poor, particularly when they are exposed to specific environments. Pure alpha iron in sheet form, not normally designated as a steel, exhibits the unusual facility for drastic cold working such as deep drawing and spinning and thus such type of relatively pure sheet iron possesses a wide utility for the manufacture of a variety of home appliances. Again, its corrosion resistance is very poor, particularly in the presence of moist air.

A most stringent requirement for protection against corrosion exists in the form of tubular products which may be utilized for hot water systems in the home, for the manufacture of hot water heaters, for heat exchange tubing involving water or other liquids, for refrigerator or dehumidifying purposes, and in the boiler industry where the water may be required to operate at elevated temperatures and pressures in the presence of dissolved air. In applications such as those just listed the corrosion problem is aggravated by the conditions of use and by the types of waters utilized in these structures. For example, hard water containing significant amounts of dissolved air, chlorides, sulfates, and compounds of alkali metals and alkaline earth metals usually as carbonates or bicrbonates is generally more drastic in its corrosive action than distilled or chemically pure water. In many cases, chemically pure water at an elevated temperature containing nearly insignificant amounts of dissolved carbon dioxide represents a corrosive agent. All of these actions are still further aggravated if these impure waters are utilized in any portion of their cycling in the presence of air. In some situations the rate of corrosion increases by orders of magnitude in the presence of air as compared with the rate of corrosion in the absence of air.

One common expedient used to provide corrosion resistance is to alloy the steel with large amounts of nickel and/ or chromium and other expensive metals, to produce the commonly known stainless steels. Alloying in this manner is a costly procedure and must be carefully controlled in order to obtain the desired analysis and a specific balancing of properties since some elements which are beneficial in one or more respects are harmful in other ways.

Another commonly practiced art is to place acoating of metal, alloy or non-metallic material over an inexpensive base metal to produce a composite with enhanced corrosion resistance as compared with the corrosion resistance of the bare base metal. Non-metallic coating materials, including resin, paint or other organic or ceramic coating materials are undesirable where the end use involves heat transfer through the coated material or where the end use involves exposure to temperatures ice which cycle or vary over relatively wide ranges such as may be encountered in a boiler or evaporator. Not only do these non-metallic coatings have lower heat transfer coefiicients than metals and coefficients of thermal expansion which differ substantially from that of the base metal (so as to produce mechanical failure of the coating based on such differences) but in many cases the corrosion resistance under the wide variety of conditions normally encountered in service is poor, even though fully coated by the non-metallics in question.

In certain installations, pure copper or copper nickel alloy tubing is utilized since it has been found that the tubing composed of such metals or alloys amply withstands the various corrosive conditions encountered in service. As a consequence, many attempts have been made to reduce the cost of manufacture of such structural components, particularly tubing, by applying coatings of copper or alloys of copper and nickel to a metallic base material such as steel.

Deposition of such metal coatings has been effected by spraying, electrolysis, dipping and by many other known coating methods, each of which possesses its own advantages and disadvantages. In the fabrication of heat exchanger tubes and refrigerator heat exchanger coils, the potential advantages of a suitably prepared copper coating deposited over the base metal have been recognized for a long time and certain phenomena which limit the usefulness of copper coatings have also been noted in the art. The problem of application of such copper containing coatings to the inside of small bore tubing aggravate the limitations of both material and process when complete corrosion resistance is required.

.Heretofore, two processes have been considered most suitable for the application of copper or copper containing alloys to the internal surfaces of a tubular structure, these being electrolysis and hot dipping in a flux comprising chiefly a copper containing compound followed by reduction at an elevated temperature. In a tube whose length may be considered substantially infinite with respect to its diameter the problems and difficulties inherent in applying a sufiiciently thick coating at a sufiiciently high rate of speed to the interior'of small bore tubing by electrolysis are obvious. As a matter of fact, the only electrolytic procedure which appears to have been found practical is the application of a copper plate by electrolysis to a fiat sheet of steel which is then subsequently rolled or otherwise formed into a tube. By virtue of the subsequent forming procedures required, this approach is expensive. In addition, defects are commonly encountered such as the presence of pinholes or other discontinuities in the copper layer as the result of uneven electrodeposition or improper cleaning of the base material. Quite often the bond between the electroplate and the base is poor and the variation of thermal expansion is suflicient to lift off whole sections of copper plate so as to expose the bare poorly corrosion resistant base materials. A single minute pinhole in an entire length of tubing permits the possibility for the setting up of a galvanic couple and the less noble metal in the pair (such as the base iron) becomes rapidly eroded, leading to failure of the article.

A large number of variations of the second method involving dipping of the base article in a molten salt containing copper have been described. This technique would be expected to exhibit the signal advantage over electrolysis that the interior of small bore tubing may be coating readily by the method and in addition the discontinuity and pinhole defect in the original coating common in electrolytically applied coatings would also be expected to be absent. In examination of the articles produced in accordance with numerous prior art proposals however, it was found that though the coatings were uni- 3 form and pinhole free they do not exhibit the desired degree of corrosion resistance. As a matter of fact, in some instances the corrosion resistance of the coated articles is actually poorer than the corrosion resistance of the base metal to which such a coating is applied.

In general, these electrodeless types of copper plating utilize variations of a common approach. In one variation an oxide of copper or a halide of copper or mixtures thereof are dispersed in organic resin, applied to the surfaces as paint and then heated to a temperature sufiicient to remove the organic portions of the composition and to cement the residual copper to the base steel, either in the presence or absence of a reducing gas such as hydrogen. In another variation, a molten salt bath is provided containing cuprous chloride as the principal constituent sometimes diluted with other compatible salts. The steel article is dipped in the molten salt bath and the cementation of copper completed either by retaining the base article in the molten bath for a sufiicient period of time or by removing the article from the bath and subsequently reducing the composition with hydrogen. Still further variation involves organic compounds of copper utilized in molten form in which the electrodeless deposition was carried out below the decomposition point of the copper organic mixture.

In order to determine the effectiveness of corrosion resistant copper containing coatings on steel structures capable of withstanding the rigors of modern requirements involving boiling water containing air, a test procedure was established for evaluation purposes. A municipal chlorinated potable water was utilized and its hardness modified to contain 69 grains of hardness by adding equal amounts of calcium carbonate and magnesium carbonate. The resulting modified municipal water contained the anticipated impurities considered normal for potable water such as calcium, magnesium, sodium and minute amounts of iron, sulfate, nitrate and chloride. The specimens containing their cladding of copper or copper containing alloy were placed in a flask half filled with such water. Then the flask was connected to a reflux condensor. The system was brought to boiling and air was bubbled continuously through the solution in contact with the clad specimen to replace air which might be removed by boiling. The water was changed every 24 hours. These tests were continued for periods of 6 to 12 weeks and the specimens were examined critically each 24 hours.

In utilization of such test and applying copper or copper alloy coatings to steel sheet stock comprised of SAE 1020 all of the coatings prepared in accordance with prior art electrolytic or molten salt dipping procedures failed in the corrosion test by exhibiting one or more aspects of erosion or corrosion of destructive character. In every instance an unsightly purplish tarnish stain developed on each of the surfaces (irrespective of the method of coating) within the first 2 or 3 hours of testing and the amount and thickness of this stain continued to increase as the test proceeded. In addition, in the majority of cases a grayish brown film, obviously oxidic in character, continued to grow at a steady rate from the surface. In every instance, in a period of about a week the erosion had proceeded through the copper plating and into the base metal and in each case where such an erosion of the coating had been completed, the base material was eroded away underneath this area and beyond the site of initial erosion to the point where it would be considered that an article made of such a composite and utilized under similar service conditions would last at best 2 to 4 weeks before perforating completely. For prior art coatings in which the copper coating was cemented on to the steel base at temperatures below the melting point of copper, the cycle of destructive processes defined in the foregoing was accelerated and in some tests the emergence of the grayish brown oxide coating and the purplish brown stain developed simultaneously. When this phe- A nomenon occurred rapidly, the final destruction of the piece was equally rapid.

The corrosion resistance of samples prepared by dipping with or without subsequent treatment in hydrogen at temperatures above ahe melting point of copper was somewhat improved as compared with cemented samples, but failure of the coating occurred in every instance. Experience with this test indicated that though the purplish brown stain developed rapidly in every test without exception irrespective of method of application, the time required for complete destruction of the piece appeared to be a function of the rate of growth of the grayish brown oxide and its time of emergence so that it would be readily visible. This time of emergence of the grayish brown oxide variedfrorn a few hours up to about 1 week and generally destructive corrosion was experienced within about 1 week after the grayish brown oxide was first observed.

The results of this examination of the teachings of the prior art appear to establish that none of the compositions of matter and techniques utilized for the applications of such compositions of matter are adequate for developing sufiicient corrosion resistance for withstanding prolonged exposure to boiling water of commercial or municipal grade in the presence of dissolved air. While the test utilized may be considered rigorous it does provide a means of evaluation and comparison which relates closely to the conditions which may be encountered in the field. To establish that the poor corrosion resistance of the copper plated articles described in the prior art is not a function of copper or copper nickel alloys themselves, similar tests were carired out with pure oxygen-free copper tubing and sheet, and pure Monel tubing and sheet. In these tests, neither the purplish discloration, nor the formation of the grayish brown oxide was observed and in each case both the pure copper speci- -rnens and the copper nickel alloy specimens withstood the test for the full 6 weeks period without any microscopic or macroscopic evidence of erosion, corrosion or destruction except for a very slight dulling of the original bright surface. It was noted however that if the pure copper contained oxygen the purplish discoloration developed rapidly but the gray brown oxide formation never developed.

It is therefore a primary object of this invention to provide steel or iron tubing in which a thin layer of a metallic copper or a copper alloy-containing composition is present as a coating on the base metal, in which this metallic copper or copper alloy composition represents a.

novel composition of matter, and in which the resulting; composite is completely corrosion resistant within thecontext of the test procedures described previously and possesses the same degree of corrosion resistance exhibited by chemically pure oxygen-free copper metal or copper alloys originally designed for corrosion resistant purposes.

It is a further object of this invention to supply a process which permits the proper application of the novel coating described in the previous sentence so that small bore tubing whose length may be considered infinite with respect to the diameter of the tubing may be coated either on the inside or on the outside of the tubing on a continuous basis in a manner depending on the eventual application of the procedure.

It is a further object of this invention to define techniques whereby strip material in flat form comprised of steel or alpha iron may be coated with the novel composition of this invention so that corrosion resistance under the conditions defined previously is complete for practical purposes.

It is another object of this invention to define novel products which may be laid over welds in a steel or iron structure so that the corrosion resistance over such welds may be equally as complete as that defined for an un-- welded structure.

It is a principal object of this invention to provide a low cost corrosion proof piping suitable for use in hot water systems or refrigerator systems whether operable at atmospheric pressures, or above, in the presence of commercial and potable waters and in contact with air.

These and other objects are achieved by the practice of this invention as defined in the following description.

The invention described in my above noted patent application comprises a novel composition of matter preferably in the form of a coating which is corrosion proof against boiling hot water in the presence of air, said composition comprising a combination of metallic copper, cuprous oxide, metallic nickel and/ or metallic cobalt and metallic iron; such coating being at least .3 mil in thickness.

Another aspect of the invention described in my above noted patent application comprises a fused coating bath consisting essentially of cuprous chloride and containing up to cuprous oxide, with or without the addition of 2 to 10% of a hydrated chloride selected from the group consisting of hydrated nickelic chloride and hydrated cobaltic chloride (CoCl -6H O) and mixtures thereof.

As described in my above noted patent application the process in which said coating and said bath are utilized comprises the following sequence of steps:

(a) Surface preparation (optional) e.g. cleaning (degrease, pickle, wash, dry).

(b) Undercoating with metallic nickel or cobalt (0ptional) e.g. by immersion of iron or steel article in dilute solution of nickel borosulfate at pH 4.5-5.5.

(0) Salt bath coating by dipping into molten salt bath at 450 C. for fraction of a minute.

(d) Heating in hydrogen atmosphere at about 1100 C. with controlled preheating and subsequent heating,

The present invention is directed to an improvement of the inventions described in the above noted patent application and particularly to a modification of steps (0) and (d) of the above sequence, it having now been found that instead of coating by dipping into molten salt, the desired coating may be obtained by applying a suitable composition to the surface to be coated, e.g. by brushing or spraying a mixture of cuprous oxide, hydrated nickelic chloride and cuprous chloride prepared by grinding the several ingredients in alcohol, and thereafter drying the coating at room temperature. It has been found also that as hereinafter described, the time the specimen is at 1100 C. and the time interval in which the temperature of the specimen is lowered from 1100 C. to at least 600 C. must not exceed certain limits.

Thus for a successful cladding procedure the maximum permissible time at 1100 C. (or other cladding temperature) is about 60 seconds and when the time at cladding temperature approaches 1 minute, the time in which the temperature of the specimen is lowered to 600 C. cannot exceed 15 seconds, whereas if the time the specimen was held at cladding temperature of 1100 C. is only 30 seconds or less then an interval of as long as 20 seconds in which to reduce the temperature to 600 C. is permissible. It is preferred that whatever the time at temperature of 1100 C. which should be as short a span as possible, dropping the temperature to 600 C. should be accomplished in a 5 to 10 second interval in a protective atmosphere.

Several techniques are available to accomplish the desired rapid cooling. The first is most effective for specimen thicknesses greater than 25 mils. This involves the use of very high frequency or ultra high frequency induction heating, at least as high as 100 megacycles and extending up to a range of 3000 megacycles. With such heating only the skin of the specimen is heated and in the 100 megacycle range and higher frequency generally specific times and temperatures.

a total thickness of about 0.5 mil or less will be heated to the measured temperature. Then by either moving the specimen out of the high frequency zone, or by shut ting off the power source the difference in temperature between the interior of the specimen and the skin is sufficient to accomplish the desired quenching within the required period of time. A second procedure, particularly effective for specimens having a gauge thickness of 25 mils or less is to move the specimen very rapidly from the heating coil area and into a zone where an increased flow of unheated hydrogen is being utilized. Thus, for example, a normal flow of hydrogen in the heating zone to maintain the desired protective atmosphere at 1100" C. would be of the order of 1 liter per minute. Adequate cooling outside the zone for the thin gauge stock is achieved by blowing cold hydrogen past the specimen at the rate of 10 liters per minute. The ideal method of quenching, whether thin or thick gauge base stock is utilized is to move the specimen directly from the heating zone into an oil bath at room temperature. This procedure is most effective, when a horizontal furnace is used for application of the copper coating, for example.

In summary, a coating consisting chiefly of metallic copper is applied to a base iron or steel surface from 'a coating bath containing halides and oxides of copper with or without hydrated salts of nickel and cobalt; thereafter the article containing the salt coating is heat treated in hydrogen at a temperature above the melting point of copper for a brief period of time for cementation purposes; the heat treated specimen is quenched to a temperature below the eutectic temperature of copper and iron which latter is of the order of 835 C. such cooling being carried out in the presence of hydrogen; whereby the base is coated with a coating of at least 0.3 mil in cross section, the exterior of which is oxygen-free as a consequence of the hydrogen treatment below the eutectic temperature of the copper iron alloy preliminarily formed; and the material at the base of the coating, attached to the iron, contains suflicient oxygen to provide an oxygen content of at least 0.1% and preferably in the range of 0.2 to 0.6%; and wherein the resulting article may or may not have nickel or an alloy of nickel and cobalt as a binding layer between the copper containing coating and iron or steel base.

In a preferred procedure, the iron or steel articles are first pickled in a l to 1 hydrochloric acid solution at 180 C. for 1 minute. To obtain the best cleaning the acid pickled material is wire brushed and repickled for 1 minute at 180 F. after which the articles are rinsed in hot water, then dipped in alcohol and then dried at room temperature. This complicated pickling procedure is required only if a nickel or nickel-cobalt flash is applied to the surface prior to the application of the copper containing coating.

The nickel flash which is placed on the iron or steel surface by electrodeless techniques is deposited simply by dipping the articles in a solution containing nickel and/ or cobalt salts buffered with boric acid at a suitable pH for In one instance, the soultion consisted of one gallon of water plus eight ounces of hydrated nickelous chloride of composition NiCl -6H O, plus boric acid. The plating solution is maintained at a temperature of F. The wirebrushed acid-pickled articles are immersed in such a bath for 1 /2 minutes, w-ater rinsed and alcohol rinsed and air dried. Nickel sulfates may be used in place of nickel chloride and a portion of the nickel chloride may be replaced with a comparable cobalt salt. The application of nickel and/or cobalt flashing on the surface results in the improvement of corrosion resistance of the over-all coating but is not absolutely necessary, nor is the drastic pickling procedure followed by wire brushing necessary, although it too is preferred.

Whether or not the steel or iron surface has been provided with an electrodeless coating of nickel or a combination of nickel and cobalt and whether or not the surface has been previously cleaned by the drastic hydrochloric acid treatment described above, the articles are immersed for 1 minute in a molten salt bath in air, said molten salt bath being maintained at 450 C. A mixture of air and steam may be bubbled through the salt bath. After the specified immersion the article is removed and allowed to air cool. A suitable salt bath consisted of a mixture containing2.5% cuprous oxide, 5.0% of hydrated nickelic chloride, and the balance of cuprous chloride. As hereinafter exemplified a similar composition may be applied to the surface without resort to a fused salt bath. The cuprous oxide content may be varied between 2 and 10% and the hydrated nickel chloride content may be varied between 2 and 10%. The lower percentages inthe ranges are preferred. Under conditions where purplish discoloration is not considered a disadvantage, the nickel salts may be omitted from the salt bath. The hydrated nickel chloride may be replaced in whole or in .part by the equivalent cobalt salt, namely, cobalt chloride hexahydrate and equivalent results are achieved.

.The coating. of small pieces may be accomplished simply by dipping. In the coating of tube stock where it is desired to coat only the interior, the steel tube stock is initially. heated in air to a temperature of 450 C. and the molten salt alsoat a temperature of 450 C. is poured into the tube and the tube allowed to drain. In the case where boththe interior and exterior of the tube is to be coated, the tube stock is pulled through a bath of the molten salt at a temperature of 450 C. so that both the interior and exterior surface of the tube are completely bathed inthe molten salt and reeled in such a manner that complete drainage is achieved. On a continuous basis such tube stock is then maintained at a temperature of 1100 C. in an atmosphere of flowing clean hydrogen in a conventional atmosphere controlled furnace, and each 'portion of the strip or tube is retained at this temperature in the atmosphere of clean hydrogen for a period not allowed to cool to room temperature in a hydrogen atmosphere. and finally removed from the furnace.

In order for the surface to exhibit the desired total corrosionresistance equivalent to that of pure metallic copper, the thickness of coating on the base steel must be a minimum of 0.3 mil and not thicker than 0.6 mil. It has been established that the presence of between 2 and 10% of cuprous oxide in the salt bath is an essential requirement for achieving this thickness. In the absence of cuprous oxide from the bath the effect of nickel in the coating is somewhat minimal.

The gray brown oxide coating which develops on corrosion testing in the water defined previously is more properly designated as iron breakthrough. This iron breakthrough represents a certain aspect of desctruction. The combination of hydrogen reduction below the first eutectic temperature plus the presence of nickel prevents not only the formation of the purplish overcast but also prevents the initiation of iron breakthrough.

The fact that the iron breakthroug is, in fact, due to precipitation of iron or an iron rich phase at the grain boundaries is clearly established by the examination of polished etched sections. Actually, this examination has led to another means for remedying the difliculty if the proper times of heating at ll C. are not followed and if the quenching is done at too slow a rate. If a sample which exhibits iron breakthrough is reheated in hydrogen or in a cracked ammonia atmosphere at 1100 C. for the proper or even too long a period of time and then rapidly quenched the iron breakthroug defect is eliminated even though the sample was originally defective.

Metallographic examination now shows that the iron has been kept completely in solution (only a single phase is exhibited) and the material is completely corrosion proof. This remedial action can be accomplished whether nickel is present or not. The double heating makes the process somewhat more expensive but can be accommodated if necessary particularly if a combination of thin and thick sections are preliminarily treated by the recommended procedure in an induction furnace with controlled atmosphere. The virtue of this accommodation is that a controlled atmosphere horizontal furnace in which the ware moves through the furnace at a relatively high rate of speed can be utilized and passed directly through a protective flame curtain into a suitable quenching bath.

Utilizing the usual induction furnace with frequencies of 30 megacycles the normal time required to bring a specimen from room temperature to ll00 C. is about 15 seconds so that a time period at 1100 C. of 30 seconds or less and preferably around 10 to 15 seconds can be accommodated easily. Using frequencies of megacycles or greater where only skin heating is accomplished, the temperature of 1100 C. at the skin can be reached in a time period of 5 to 10 seconds so that again maintenance of the skin at 1100 C. for not more than 10 to 30 seconds is easily accomplished and as stated before under these conditions and if the gauge of the underlying metal is greater than 25, simply moving the part from the induction heated zone to a unheated zone sometimes is sufficient to accomplish the quenching in the desired short span of time while still in the controlled atmosphere, providing a sufiicient mass of underlying cold metal is available.

Instead of resorting to the drastic quenching schedule described above elimination of all evidences of corrosion may be achieved by adding nickel chloride or cobalt chloride to the mixture of cuprous chloride and cuprous oxide. In the presence of the cuprous oxide and in accordance with the preferred process a coating thickness of 0.4 to 0.5 mil was regularly obtained, no purple tarnish was ever observed, and the iron breakthrough was eliminated entirely. For all practical purposes, the corrosion resistance under the boiling water aerated test appeared to be permanent. A slight dulling of the surface did develop after weeks. When the combination of cuprous chloride, cu rous oxide and post reduction at 700 C. for 2 minutes to remove exposed cuprous oxide was utilized even this dulling was eliminated.

The presence of nickel reduces any tendency to iron breakthrough and the higher the concentration of nickel the greater the protective aspect of the presence of nickel. in the absence of nickel the tendency to iron breakthrough may be due to two causes. The first is the solubility of iron in molten copper which may extend to several precent particularly if the molten copper is held in contact with the steel for too long a period and the second is the tendency for this iron to precipitate out as metallic iron, particularly at the grain boundaries if a solution ofiron in copper is cooled slowly thru the freezing point. Simply increasing the amount of nickel so as to minimize the effect of iron as completely as possible, holds the iron in solution possibly as a copperiron-nickel alloy but more probably as an iron-nickel alloy dissolved in copper. However, such a practice would not be used when it is desired to have a relatively low nickel containing surface for corrosion protection of iron in order to minimize costs.

As pointed out previously, drastic acid pickeling and nickel flashing represents an advantage but these are not required to produce commercially acceptable results. The coating bath itself in the low temperature immersion stage evidently is a. drastic pickling operation and the effectiveness of its utility is evidenced by the fact that it works equally well across flat surfaces as it does across weldments. In order to establish the effectiveness of the addition of nickel and cobalt to the coating itself samples of pure copper which contained proportions of cuprous oxide in its composition were subjected to the boiling water test and again the purplish discoloration previously described was obtained. The effect of the addition of nickel on the elimination of such purplish discoloration was dramatic.

In order to establish the chemical composition of the coating which defines one aspect of this invention an immersion was made in a bath containing 92.5% cuprous chloride, 5.0% nickelic chloride hexahydrate and 2.5% cuprous oxide. After treatment in hydrogen through the preferred temperature cycling an analysis of this coating showed that it contained 1.43% nickel, 3.26% iron and 0.3% oxygen, the balance being copper.

It was found that in order to achieve the desired thickness of coating through the addition of cuprous oxide in the first immersion bath that the final coating must contain an average of at least 0.1% oxygen and preferably from 0.2 to 0.6% oxygen in order to produce the desired results while still maintaining a useful degree of flexibility for working the finished product without delamination of the coating from the surface.

The following specific examples will serve to illustrate the invention and are not intended to be taken as limitative.

Example I A steel speciment comprising two sections, each 25 mils in thickness joined together by a weldment, was first irnmersed in a 1 to 1 hydrochloric solution for 2 minutes at 180 F. and then wire brushed to remove any oxide not removed by the acid pickle. After water washing and alcohol rinsing and drying, the steel specimen containing the weldmen was immersed in a molten salt bath comprising a mixture of 92.5 parts of anhydrous cuprous.

chloride, 2.5 parts of red cuprous oxide, and 5.0 parts of nickel chloride hexahydrate melted and maintained at 450 C. Immersion was for a period of 1 minute, after which the specimen was removed and allowed to cool to room temperature. The specimens were suspended vertically in a hydrogen atmosphere flowing at the rate of 1 liter per minute inside an induction coil operated at a frequency of 30 megacycles through which device the specimen was brought to a temperature of 1100 C. over a time period of 15 seconds and maintained at this temperature for 30 seconds. The temperature was thereafter lowered to 600 C. in 15 seconds by shutting oif the power input to the induction heater, and increasing hydrogen flow to liters per minute. The specimen was then cooled to room temperature in about 3 minutes While still surrounded by the hydrogen atmosphere, after which it was removed from the furnace.

Using municipal water containing 60 grains of hardness as the result of the deliberate addition of equivalent amounts of calcium and magnesium carbonates, the specimen was corrosion tested in the solution at the boiling point under reflux conditions for a period of 6 weeks. During the test air was bubbled continuously through the water in contact with the specimen. At the end of 6 weeks the bright original surface was still retained with no evidence of corrosion or tarnish.

Example II The composition and techniques defined in Example I were again utilized except that the welded specimen was given no treatment whatsoever prior to immersion in the molten cuprous chloride-cuprous oxide-nickel chloride salt bath at 450 C. After the treatment was complete it was found that the copper coating had flowed under the oxide coating covering the steel and the weld and that the oxide coating adhered to the outer surface of the copper. Wire brushing removed the oxide coating exposing a bright shiny surface again and corrosion testing of such bright exposed surface under the conditions previously defined demonstrated that protection was com- 10 plete. In a variation of this example the oxide coating was not removed by a wire brushing but was found to be substantially all removed by the turbulent action of the boiling water in the corrosion test.

Example III A sheet of 25 mil SAE 1020 steel was pickled in 1 to 1 hydrochloric acid at 180 F. for 1 minute, after which the specimen was water rinsed, alcohol dipped and dried at room temperature. A nickel flash was then applied by immersing such pickled and clean sample in a bath containing 8 ozs. per gallon of nickel chloride hexahydrate and 5 /2 ozs. per gallon of boric acid at a temperature of 160 F. for seconds after which it was water rinsed, alcohol rinsed and air dried. This specimen was then immersed in a molten salt bath comprising 92.5 parts of anhydrous cuprous chloride, 2.5 parts of red copper oxide and maintained at a temperature of 450 C. in air for a period of 1 minute and then allowed to air cool. Thereafter, the specimen was treated in a flowing hydrogen atmosphere at the temperatures described in Example I. Corrosion testing showed no evidence of iron breakthrough and no purple tarnish and measurement of the coating thickness indicated that a coating of the order of 0.4 to 0.5 mil had been achieved. After the corrosion testing was carried out for a period of 12 weeks the formation of purple tarnish started to become evident.

Example IV The same as in Example III except the iron base in the preliminarily pickled condition was utilized without the nickel immersion coating. Again, the sample withstood the corrosion test conditions without any major defect except for the emergence of purple tarnish in the 4th week of corrosion testing.

Example V A pure iron base without nickel flashing was utilized and the molten salt bath was comprised of 90 parts of anhydrous cuprous chloride, 5 parts of red copper oxide and 5 parts of nickel chloride hexahydrate. By following the heat treatment and immersion described in Example I,

a coating thickness of 0.5 mil was attained. The specimen was subjected to corrosion testing and no evidence of either iron breakthrough or purplish tarnish was obtained after 12 weeks of corrosion testing.

Example VI A pure iron base was deliberately oxidized by heat treating in air at 600 C. prior to the application of the dip bath. The evenly oxidized iron base without nickel flashing was treated in a bath comprised of parts of anhydrous cuprous chloride and 5 parts of red cuprous oxide. Again, the appearance of the coat and adhesion of the coat to the base was excellent and a thickness in the range of 0.4 to 0.5 mil of coating was obtained. In the water test, no evidence of iron breakthrough was obtained and while a slight developement of purplish coating.

was produced after about 6 weeks, all further corrosion appeared to cease after the emergence of this purplish coatmg.

Example VII A pickled steel specimen was immersed in a molten bath containing 94% anhydrous cuprous chloride, 2% tin as stannous chloride and 4% manganese as manganous chloride. The resistance to the boiling water test was poor, showing both the purplish overcast and the iron breakthrough and the thickness of the coating achieved in accordance with the schedule given in Example I was 0.1 to. 0.15 mil.

Example VIII To the clipping composition given in Example VII, 5 parts of red cuprous oxide was added. In this case the corrosion resistance was good except for the delayed emergence of the thin purplish coating. The thickness of the coating was in the range of 0.4 to 0.5 mil and after 12 weeks of corrosion testing, no further development of corrosion beyond the initial emergence of the thin purplish surface was evident.

In order to form alloys with the copper coating, the fused salt bath may be modified by the addition thereto of suitable amounts, e.g. 2-5 by weight of stannous chloride, zinc chloride, manganous chloride, chromous chloride and mixtures thereof, provided sufficient cuprous oxide is present to yield the necessary coating thickness.

Example IX A circular cross section steel specimen 1" in diameter and 3" in length was pickled and treated with the molten salt bath by the procedures and compositions as defined in Example I. The specimen was then suspended vertically in a hydrogen atmosphere flowing at the rate of liters per minute and brought to a surface temperature of 1100" C. over a time period of six seconds and maintained at this temperature for seconds through the medium of heating inside an induction coil operated at 1000 megacycles. After this treatment the specimen was then dropped completely below the high frequency coil and it was found that the surface temperature dropped below red heat (i.e. below 600 C.) in about 9 seconds. The specimen was retained in this non-heating area for about 3 minutes while still surrounded by the hydrogen atmosphere after which it was removed from the furnace. On applying the corrosion procedure as defined by Example I, it was found that no iron breakthrough was experienced. The original bright surface was retained with no evidence of corrosion or tarnish.

Example X A steel specimen comprising a bar 3" in length and 1" in diameter was pickled and treated in the molten salt bath as in Example I. This specimen was then suspended vertically in a hydrogen atmosphere flowing at the rate of 1 liter per minute and brought to a temperature of 1100 C. inside a coil operating at 30 megacycles. The time period required for achieving this temperature was 40 seconds and the specimen was maintained at this temperature for 2 minutes. The power was shut off and 70 seconds was required for the temperature to reach 600 C. The specimen was then cooled to room temperature in about 3 minutes while still surrounded by the hydrogen atmosphere, after which it was removed from the furnace. Corrosion testing of this specimen prepared in this manner showed that iron breakthrough existed and that the specimen could not be considered corrosion proof.

Thereafter, the specimen was suspended from a rack and said rack containing the specimen was placed in a horizontal continuous furnace containing a flowing protective atmosphere of cracked ammonia. The exit end of the furnace was covered with a flame curtain and the hot zone was maintained at 1100 C. The specimen was passed through this continuous furnace which had a length of track and speed of travel such that the specimen was retained at li -00 C. for 2 minutes. Immediately after passing through the end of the hot zone which was the exit end of the furnace protected by the flame curtain the hot specimen was quenched in a bath of No. motor oil. The specimen was then checked for corrosion resistance by'the technique defined in Example I and the end of six weeks the bright original surface was still retained with no evidence of corrosion or tarnish or of iron breakthrough. This treatment establishes the importance of high speed cooling under controlled and ambient conditions for retention of the corrosion resistance properties of the coating.

Because with repeated use, the salt baths previously described tend to accumulate iron and to thereby lose some of their initial activity it has been found desirable to substitute in place of the salt bath coating procedures described in Examples I to X, a procedure in which the base is coated by a paint in a v'aporizable vehicle. Such a procedure is illustrated in Example XI which follows:

Example XI A steel specimen comprising two sections, each mils in thickness joined togther by a weldment, was first immersed in a 1 to 1 hydrochloric solution for 2 minutes at 180 F. and then wire brushed to remove any oxide not removed by the acid pickle. After water Washing and alcohol rinsing and drying, the steel specimen containing the weldment was painted with coating composition containing 92.5 pants of anhydrous cuprous chloride, 2-5 parts of red cuprous oxide, and 5.0 parts of nickel chlo- 15 ride hexahydrate prepared by grinding the several salts in 50 parts of ethanol to produce an intimate mixture thereof. The amount of alcohol is not critical and should suffice to slurry the several salts so that a continuous coating of salt mixture is readily deposited on the ferrous article 'by brushing or spraying and then permitting the coating to dry in air.

In the above description and examples, all parts are by weight unless otherwise indicated.

I claim:

1. A process of providing a ferrous base material with an adherent corrosion-proof coating at least about 0.3 mils in thickness on at least one surface of said material, said process comprising:

coating the surface of said material with a mixture consisting essentially of between 2% and 10% by weight cuprous oxide and the balance substantially all cuprous chloride;

thereafter heating the coating base material in a hydrogen atmosphere at about 1100 C. for not over 1 minute; and

thereafter quenching said heated material to a temperature below 700 C.

2. A process of providing a ferrous base material with an adherent corrosion-proof coating at least 0.3 mil in thickness on at least one surface of said base material comprrsmg:

cleaning the surface of said base material which is to be coated; subjecting the cleaned surface of said material to the action of a fused salt mixture consisting essentially of between 2% and 10% by weight cuprous oxide and the balance substantially all cuprous chloride, while at a temperature of about 450 C.;

thereafter heating the coated base material in a hydrogen atmosphere at about 1100 C. for not over one minute;

thereafter cooling said material to a temperature below about 700 C. in between 5 and 30 seconds;

and then cooling the material to room temperature while in a hydrogen atmosphere.

3. A process of providing a ferrous base material with an adherent corrosion-proof coating between about 0.2 and 0.4 mil in thickness on at least one surface of said base material comprising:

cleaning the surface of said base material which is to be coated; depositing a thin film of a metal selected from the group consisting of nickel, cobalt and mixtures of nickel and cobalt onto said cleaned surface;

subjecting the surface of the resulting coated article to the action of a fused sa-lt mixture consisting essentially of between 2% and 10% by weight cuprous oxide and the balance substantially all cuprous chloride, while at a temperature of about 450 C. for a short time interval;

thereafter heating the salt coated surface of said material in a hydrogen atmosphere to about 1100 C. for not more than one minute;

thereafter rapidly cooling said heated material to about 600 C. in a hydrogen atmosphere;

and then further cooling the material to room temperature while in a hydrogen atmosphere.

4. A process of providing a ferrous base material with an adherent corrosion-proof coating at least about 0.3 mil in thickness on at least one surface of said base material comprising:

subjecting the surface of said material to the action of a fused salt mixture consisting essentially of between 2% and 10% by weight cuprous oxide, between 2% and 4% by Weight of nickelic chloride and the balance substantial-1y all cuprous chloride, while at a temperature of about 450 C. for a short time interval;

thereafter heating the coated base material in a hydrogen atmosphere at about 1100 C. for not longer than one minute;

thereafter rapidly cooling said material to about 700 C. within an interval of 15 seconds;

and then cooling the material to room temperature while in a hydrogen atmosphere.

5. The process of claim 4 wherein the salt bath contains between about 2% and 5% by weight of at least one additional chloride of a metal which alloys with copper and selected from the group consisting of tin, zinc, manganese and chromium.

6. A process of providing a ferrous base material with an adherent corrosion-proof coating at least about 0.3 mil in thickness on at least one surface of said material, said process comprising: subjecting the surface of said material to the action of a fused salt mixture consisting essentially of between 2% and 'by weight cuprous oxide and the balance substantially all cuprous chloride, while at a temperature of about 450 C.; thereafter heating the coated base material in a hydrogen atmosphere at about 1100 C. for not over 1 minute; and thereafter guenching said heated material to a temperature below 7. A salt bath suitable for the deposition of a copper containing coating on a ferrous base material consisting essentially of between 2% and 10 weight percent of cuprous oxide and the balance substantially all cuprous chloride.

8. A salt bath suitable for the deposition of a copper containing coating on a ferrous base material consisting essentially of about 2.5% by weight of cuprous oxide, 5% of nickelic chloride and the balance substantially all cuprous chloride.

9. A salt bath suitable for the deposition of a copper containing coating on a ferrous base material consisting essentially of between 2 and 1 0% by weight of cuprous oxide, between 2% and 10% by Weight of a chloride selected from the group consisting of nickelic chloride, cobaltic chloride and mixtures of nickelic and cobaltic chlorides and the balance substantially all cuprous chloride.

10. A salt bath suitable for the deposition of a copper containing coating on a ferrous base material consisting essentially of between 2 and 10% by weight of cuprous oxide, between 2% and 10% by weight of nickelic chloride and the balance substantially all cuprous chloride.

References Cited by the Examiner UNITED STATES PATENTS 141,132 7/1873 Gauduin 117-130 1,197,616 9/1916 Eldridge 117-130 2,398,738 4/1946 Gilbert 117-130 2,737,463 3/ 1956 Lawton et al. 117-46 X 2,872,348 2/1959 Eubank 117-130 X FOREIGN PATENTS 23,576 1914 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. RALPH S. KENDALL, Examiner. 

1. A PROCESS OF PROVIDING A FERROUS BASE MATERIAL WITH AN ADHERENT CORROSION-PROOF COATING AT LEAST ABOUT 0.3 MILS IN THICKNESS ON AT LEAST ONE SURFACE OF SAID MATERIAL, SAID PROCESS COMPRISING: COATING THE SURFACE OF SAID MATERIAL WITH A MIXTURE CONSISTING ESSENTIALLY OF BETWEEN 2% AND 10% BY WEIGHT CUPROUS OXIDE AND THE BALANCE SUBSTANTIALLY ALL CUPROUS CHLORIDE; THEREAFTER HEATING THE COATING BASE MATERIAL IN A HYDROGEN ATMOSPHERE AT ABOUT 1100*C. FOR NOT OVER 1 MINUTE; AND THEREAFTER QUENCHING SAID HEATED MATERIAL TO A TEMPERATURE BELOW 700*C. 